Heart failure is a debilitating disease in which progressive decrease in function of the heart results in inadequate blood flow to the tissues and organs of the body. The heart may lose propulsive power because of a decrease in ability of the cardiac muscle to contract, impaired filling of the chambers of the heart or both. Often, it is the ventricles that do not adequately eject or fill with blood over the cardiac cycle, and the valves regulating blood flow become leaky, allowing regurgitation or back-flow of blood. The impairment of arterial blood flow deprives the body's organs of oxygen and nutrients. Additionally, a cascade of maladaptive neurohormonal responses may take place, bringing about fluid and sodium retention, autonomic imbalance, vascular and cardiac remodeling and possibly dilatation of the heart. Fatigue, weakness and the inability to carry out daily tasks may result. Not all heart failure patients suffer debilitating symptoms immediately. Some may live actively for years. Yet, with few exceptions, the disease is relentlessly progressive.
Nearly half of those with heart failure suffer from diastolic dysfunction, also called diastolic heart failure (DHF) or heart failure with preserved ejection fraction (HFpEF), wherein systolic function is generally preserved but diastolic function is compromised. Diastolic dysfunction refers to an abnormality in the ability of the heart to fill during diastole, which is the phase of the cardiac cycle when the ventricles relax and fill with blood prior to contraction. With DHF, an assessment of ventricular filling is particularly important for managing patients. There may be impaired diastolic function in many patients with systolic heart failure as well, which may help to understand the variable response of these patients to therapies aimed to treat systolic dysfunction. Currently, noninvasive assessment of diastolic function is typically done using echocardiography by detecting and examining E-waves and A-waves in conjunction with an electrocardiogram (ECG.) The E-wave corresponds to the flow of blood across the mitral valve during early diastole; the A-wave corresponds to flow of blood across the mitral valve during atrial contraction near the end of ventricular diastole. See, for example, Barold et al, Europace 2008; 10 (suppl 3):iii88.
Echocardiographic assessment of ventricular filling in diastole is useful both for diagnostic purposes as well as for guiding programming of cardiac devices. However, echocardiography can be time consuming and operator dependent, and is not often used in clinical practice. In fact, there is a lack of evidence suggesting beneficial impact of biventricular pacing devices in patients with primarily diastolic dysfunction. The utility of a device might be increased if it could be used to optimize diastolic function. Thus, techniques for more easily and reliably assessing ventricular filling would be valuable for both in-clinic and ambulatory adjustment of pacing control parameters (such as atrioventricular delay (AVD) parameters) within pacemakers, CRTs or other cardiac rhythm management devices (CRMDs), as well as for early detection of changes in the cardiac condition. It is to these ends that the invention is generally directed. In particular, systems and methods are provided for use with CRMDs to exploit dynamic cardiogenic impedance (i.e. Zc) signals to estimate diastolic function, specifically diastolic flow characterization of passive early filling (equivalent to the echocardiographic E-wave) and atrial kick (equivalent to the echocardiographic A-wave.)